In normal headphone listening, when the listener rotates his head, the sound scene rotates accordingly. In a 3D audio context, headtracking defines the monitoring of an orientation of the listener's head. This orientation information may then be used to control spatial processing such as 3D audio rendering to compensate for head rotations. In employing head rotation compensation the sound scene presented to the listener can be made stable relative to the environment.
Stabilization of the sound scene produces several advantages. Firstly, by employing headtracking the perceived 3D audio quality of a spatialization system may be improved. Secondly, by employing headtracking new 3D audio solutions can be developed. For example virtual and augmented reality applications can employ headtracking.
3D audio processing is typically performed by applying head related transfer function (HRTF) filtering to produce binaural signals from a monophonic input signal. HRTF filtering creates artificial localization cues including interaural time difference (ITD) and frequency dependent interaural level difference (ILD) that auditory system uses to define a position of the sound event.
However, localization performance of a static (in other words head motion independent) 3D audio spatialization system has certain limitations. An auditory event is said to be localized to a so-called “cone of confusion” if the ILD value is the same for all positions, but the frequency dependent ILD varies. As the ITD cue in the cone is ambiguous, the listener will have difficulty in discriminating sounds based only on their spectral characteristics. As a result, front-back reversal is a common problem in 3D audio systems.
Head motion provides an important aid to help to localize sounds. By moving the head the ITD between the ears can be minimized (which can be considered to be equal to switching to the most accurate localization region). In all cases in which localization is anomalous or ambiguous, exploratory head movements take on great importance such as indicated in Blauert, J., “Spatial Hearing: The Psychophysics of Human Sound Localization”, (rev. ed.), The MIT press, 1996.
Thus, headtracking gives the listener a possible way to use head motion to improve localization performance of the 3D audio system, and especially for front-back reversals.
Modern microelectromechanical system (MEMS) or piezoelectric accelerometers, gyroscopes and magnetometers are known to provide low cost and miniature components that can be used for orientation tracking. This tracking is based on absolute measurements of the direction of gravity and Earth's magnetic field relative to the device. Gyroscopes provide angular rate measurements which can be integrated to obtained accurate estimates of the changes in the orientation. The gyroscope is fast and accurate, but ultimately the integration error will always cumulate, so absolute measurements are required. Magnetometers, unfortunately, suffer from significant calibration issues, of which only some have been solved. In some augmented reality systems which contain a camera, the optical flow of the camera system can also be used for headtracking. In many occasions headtracking is performed by a fusion of many methods.
Spatial audio processing, where audio signals are processed based on directional information may be implemented within applications such as spatial sound reproduction. The aim of spatial sound reproduction is to reproduce the perception of spatial aspects of a sound field. These include the direction, the distance, and the size of the sound source, as well as properties of the surrounding physical space.